Ultrasonic Tankless Coil Cleaning

There's a guy here on Long Island who has been using an "ultrasonic" gadget to clean boiler boils for several decades.

He hooks up a wooden box wrapped with tape between an air compressor and any hot water faucet. He plugs in the box, which he calls an ultrasonic transducer, and turns on the compressor. This does a great job of cleaning the coil. Pounds of mung come out of any open faucet.

Any idea how this works?